Ionothermal Synthesis of N-Deficient Graphitic Carbon Nitride with Enhanced Photocatalytic Hydrogen Evolution Activity

Graphitic carbon nitride (g-C3N4) was synthesized by thermal polymerization method using urea and dicyandiamide as raw materials. Then, g-C3N4 was treated at different temperatures by two post-heat treatment methods: direct secondary calcination and molten salt ionothermal post-heat treatment, respectively. As s results, different nitrogen-defected carbon nitride materials CN and CNS were obtained. The prepared samples were characterized and analyzed via X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance spectrum (UV-Vis DRS), transmission electron microscopy (TEM), and so on. The effect of different heat treatment temperatures and heating methods on the microstructure of catalysts were discussed. The photocatalytic properties of catalysts were tested by H-2 production from water splitting under visible light irradiation. The results showed that ionothermal post-heat treatment method was more conductive to interlayer polymerization of g-C3N4 than direct calcination, and high crystallinity materials were obtained. Nitrogen defects were introduced into the in-plane heptazine polymerization accompanying the production of terminal cyano groups. The strong electron absorption of cyano could optimize the charge density distribution and enhanced the charge fluidity. In addition, molten salt heat treatment increased porosity and expanded the light absorption range of CNS samples. CNS-500 prepared under 500 degrees C ionothermal post-heat treatment exhibited excellent photocatalytic hydrogen evolution activity, which was 3.84 times as high as that of the catalyst prepared by direct post-heat treatment at the same temperature.